The present invention concerns the preambles of the independent claims. It is therefore concerned with the fabrication of photovoltaic solar cells.
In the field of photovoltaic solar cells, the aim is usually to deliver a given power output at the lowest possible price. This demands both high efficiency and minimal production costs. The raw material can be a major contributor to cost. It is therefore desirable, whenever possible, to use multicrystalline silicon rather than high-purity single-crystal Czochralski silicon. A disadvantage of the use of multicrystalline silicon, however, is that it contains many efficiency-lowering defects. These include not only geometric defects in the structure of the crystal lattice, such as dislocations and crystal boundaries, but also incorporated foreign atoms that settle preferentially at the crystal boundaries.
To reduce the negative impact of such foreign atoms, etc., it has been proposed to neutralize the detrimental effects of at least the electronically charged defects by introducing atomic hydrogen. It is important that this atomic hydrogen not be allowed to remain at the surface of the solar cell being fabricated, but rather that it penetrate into the volume interior in order to reach the immediate vicinity of the defects to be passivated.
A number of techniques for achieving hydrogen passivation are known from the prior art. For example, it has been proposed to implant high-energy hydrogen ions in the surface region of a silicon wafer and then drive them into the volume interior by thermal means. It has further been proposed to expose the silicon wafer to a hydrogen atmosphere at temperatures that are selected to be sufficiently high, 700xc2x0 C., for example, so that the molecular hydrogen dissociates and can then diffuse into the wafer. It has also been proposed to expose the samples to a hydrogen plasma that is generated capacitively or inductively directly on the silicon wafer, with no intermediary. It has further been proposed to carry out the passivation in combination with antireflex coating of the solar cells. In this case, a hydrogen-containing silicon nitride film can be deposited by PE CVD; the hydrogen atoms present in the superficial, hydrogen-containing silicon nitride film then pass into the volume of the solar cell during a subsequent processing step.
Also to be cited is the paper xe2x80x9cDetailed study on Microwave Induced Remote Hydrogen Plasma Passivation of Multicrystalline Silicon,xe2x80x9d by M. Spiegel, P. Fath, K. Peter, G. Willeke and E. Bucher, in the 13th EC-PVSEC, Nizza, 1995, pp. 421 et seq. There, it is proposed to generate a hydrogen plasma by microwave irradiation remotely from the location of the solar cells to be processed and then to guide the plasma to the solar cells. As a rule, however, the known techniques are slow, require high technical expenditure for their industrial implementation, and/or are incompatible with typical present-day production methods. The ability afforded by the prior art to fabricate a high-efficiency solar cell at low cost is therefore limited.
The object of the present invention is to provide an innovation for industrial application, and particularly, but not exclusively, both to improve the opportunities for cost-effective manufacture of high-efficiency solar cells and to extend the methodologies gained in this area to other fields of semiconductor fabrication.
The means of accomplishing this object is claimed independently. Preferred embodiments can be found in the dependent claims.
The invention therefore proposes a process for treating semiconductors, wherein material is deposited on a semiconductor and passivation is performed by means of hydrogen plasma, the material being deposited by low-pressure CVD and the hydrogen passivation being effected by feeding in a hydrogen plasma induced remotely from the partially processed semiconductor.
In view of the differences between the process conditions of PE CVD and those of low-pressure CVD, despite the known passivation of solar cells made by the PE CVD process, it was surprising to one skilled in the art that passivation by means of hydrogen induced by microwave remotely from the location of the partially processed solar cells should bring about a significant improvement in efficiency in the low-pressure process as well, yet without substantially increasing the cost of the process.
Preferred choices for use as the semiconductor are silicon semiconductors or semiconductors containing substantial proportions of silicon. The process according to the invention is especially preferable for the fabrication of solar cells from reasonably-priced silicon substrates. Good improvements are achieved in solar cells made from multicrystalline silicon substrates, in particular; but significant improvements are also possible, for example, in the case of poorer-quality and hence more defect-ridden single-crystal silicon substrates and in the case of thin-film solar cells.
The partially processed solar cells can be heated for at least part of the time during the hydrogen passivation; this can be accomplished by means of thermal radiation from an IR light source and/or a resistance heater. Such active heating during the hydrogen passivation is preferable over keeping the temperature unchanged from a previous step, because in this way the ideal process parameters can be set.
In a preferred embodiment, the low-pressure CVD with which the hydrogen passivation according to the invention is combined is carried out to precipitate silicon nitride. This precipitation reaction typically occurs at about 750xc2x0 C. Here, the actual hydrogen passivation according to the invention, effected by means of remotely induced hydrogen plasma, which typically proceeds for, e.g., 30 min at 350xc2x0 C., can be shortened by having the passivation take place both during precipitation and during the heating and/or cooling phase. The reaction time for the passivation step per se can be shortened still further if the LP CVD (low-pressure CVD) is followed by a screen-printing and firing process and/or another contact firing process, since this also enables the hydrogen to diffuse deeper into the cell. The passivation is therefore performed at least in part during the change in temperature necessary for performing one or more other process steps, and furthermore, at least some of the hydrogen passivation is simultaneously performed during at least one other treatment step.
This basically makes it possible to perform volume passivation by means of a remotely induced hydrogen plasma without increasing the solar-cell processing time. The additional cost of the solar cell process according to the invention therefore depends solely on the equipment costs and additional operating costs occasioned by the hydrogen gas or hydrogen-gas mixture and the energy consumption necessary for plasma induction.
The hydrogen plasma is preferably generated by microwave irradiation, since this permits especially good control and/or regulation of the radiation intensity. The plasma generation can be carried out in a manner known per se in the presence of a nonreactive gas, especially a noble gas, especially helium. In particular, this reduces self-reactions in the plasma from the site of plasma induction to the location of the partially processed solar cells, thereby increasing the efficiency of the plasma treatment.
It is additionally preferred if, for at least a portion of the passivation phase, the hydrogen plasma is brought into contact with the partially processed solar cells in the presence of other gases. In this case, the plasma chemically activates the gases such as NH3, SiH4, SiH2Cl2 used to trigger precipitation, which increases the proportion of atomic hydrogen at the sample location and simultaneously accelerates the process of precipitation of, for example, SiN.
The solar cells can, of course, be advanced through a system continuously in small units, for example lying horizontally or standing vertically in groups, and processed in this way; however, it is preferable if a batch-type process is performed on a plurality of simultaneously processed wafers, since the process conditions can be controlled especially well with this approach.
The subject matter of the invention further includes a device for fabricating solar cells from a multicrystalline silicon substrate and comprising a reaction chamber for processing the solar cells, and a source of passivating hydrogen, said source of passivating hydrogen comprising a microwave generator for the microwave induction of plasma and being disposed remotely from the reaction chamber, and the reaction chamber being designed for the performance of low-pressure CVD. It can be provided, in particular, that the system is elongated for treating a large number of wafers simultaneously and the hydrogen is injected transversely to the longitudinal axis, there being provided a plurality of hydrogen-plasma injection openings, to which a plurality of hydrogen-plasma-inducing microwave arrangements is preferably assigned. The performance of the process according to the invention in such a system can be substantially automated by suitable process control.